Interpretive Summary: Nitrogen (N) from agricultural activities and its transport into surface water systems has been linked to accelerated growth of algae and weeds and a decline in water quality. The impact of N movement into surface water systems due to intensive agricultural activity is especially apparent in the North Carolina Coastal Plain region. Rivers and streams in this region include in-stream wetlands (ISW) in their drainage systems prior to discharge in North Carolina coastal waters. These ISW function as natural filters for N by slowing outflows and allowing for plants, microorganisms, and sediment to assimilate and store this nutrient. In some cases, however, storms can increase water flows beyond the ISW ability to filter N. We measured outflows and stream water N concentrations from an ISW over a nine-yr period to better understand how storms influence N export dynamics. Over the nine yrs, the majority of the N leaving the ISW was not accelerated by storms. Most of the N was exported during the winter period when plant and microbial N uptake processes were at their minimum. The three hurricane events in 1999 resulted in accelerated N exported because of the substantially higher flow volumes. Our results show that, for the most part, the ISW can reduce downstream water N loads.

Technical Abstract:
In-stream wetlands (ISW) intercept stream water and act as nitrogen (N) sinks influencing nitrate and ammonium export to downstream aquatic ecosystems. Nitrogen assimilation and storage by ISW, however, can be affected by storm flows, seasonal changes in water quality or shifts in N pools, resulting in N releases. This study evaluated relationships between ISW outflow events, seasonal characteristics, and sediment N pools that influence nitrate and ammonium concentrations and exported N loads. The ISW was located at the outlet of an agriculturally intensive 425-ha watershed in Duplin Co., North Carolina. Outflow N concentrations and total outflow (Qt) were monitored for nine consecutive yrs. Total outflow was further separated into direct runoff (Qdr) and base flow (Qbf) episodes and used to calculate N mass loads during these events. Additionally, sediment total Kjedhal N (TKN), ammonia and nitrate concentrations in sediment pore water were used to examine water column-sediment pore water N shifts. Nitrate releases were linearly correlated with wetland Qt, Qdr, and Qbf events with higher mass loads released during the winter. Over the nine yrs, Qb events contributed to most of the N were exported from the ISW. In 1999, three successive hurricane events grossly accelerated outflow Qt and N releases. Both sediment TKN and pore water N concentrations shifted due to hydrologic disturbances from storm events. These results imply that this ISW ability to store N was highly dependent on Q characteristics and seasonal influences on N assimilation processes.